It is well known to case harden a steel workpiece by induction heating and quenching the workpiece. In induction heating, a rapidly fluctuating magnetic field created by alternating current in an electrically conductive coil around the work piece induces electric current flow in the work piece near the surface thereof. The induced current resistively heats a thin layer of the work piece to a high temperature which depends on the power density and the duration of the induction heating cycle. Where the surface of the workpiece is plain, the current flow and the high temperature layer closely follow the surface so that quenching produces a uniformly thin, hard case. Where the surface of the workpiece is undulating, e.g. gear teeth, the induced electric current flows across the roots of the undulations instead of following the surfaces thereof so that the high temperature layer and the corresponding hard case penetrate to the roots of the undulations instead of following the surface thereof. Because of this characteristic, case hardening of rack bars for motor vehicle rack and pinion steering gears, having both plain and toothed portions, includes induction heating of the plain portions and conduction heating of the toothed portions. The combination of induction heating and conduction heating is more expensive than induction heating alone and also requires more extensive post-hardening straightening of the rack bar to achieve conformity with rigorous dimensional specifications.